Focus detection pixels, each comprising a micro-lens and a pair of photoelectric conversion units disposed to the rear of the micro-lens, are arrayed on a predetermined focal plane of a photographic lens. Via this array, a pair of image signals corresponding to a pair of images formed with a pair of focus detection light fluxes passing through an optical system are generated. The focusing condition (a defocus amount indicating the extent of defocus) at the photographic lens is determined by detecting an image shift amount (phase difference), i.e., the extent of image shift manifested by the pair of image signals. A focus detection device engaged in such an operation is known in the related art as a focus detection device adopting the split-pupil phase detection method.
Focus detection is executed concurrently as a live view image display is provided by reading out focus detection pixel signals and image-capturing pixel signals over predetermined specific frame time intervals from an image-capturing element (image sensor) configured with focus detection pixels such as those described above and image-capturing pixels disposed thereat in combination. In addition, the signals from the focus detection pixels output in correspondence to a plurality of previous frames are stored frame by frame. If the signals from the focus detection pixels for the most recent frame fail to achieve a sufficient output level and thus the focusing condition cannot be detected in conjunction with these signals alone, the focus detection pixel signals stored over the plurality of past frames are added together for temporal accumulation. There is a focus detection device known in the related art (see patent literature 1) that executes focus detection based upon focus detection pixel signals with the output level thereof raised through such measures.